Acoustic thickness measurements using gas as a coupling medium

ABSTRACT

An acoustic apparatus adapted to operate in a gas filled space from a first side of an object to be measured for making a non-contact thickness measurement thereof includes an electro acoustic transducer, a transceiver coupled with the electro acoustic transducer and adapted to excite electro acoustic transducer to output an acoustic signal towards the object to be measured and receive an acoustic response signal therefrom, and a signal processor adapted to process the response signal and determine a thickness of the object. The electroacoustic transducer has a transducer-to-gas acoustic interface, and the transceiver is adapted to operate the electroacoustic transducer so as to emit into a gas filled gap an acoustic broad band pulse towards the object and to receive an acoustic resonance response signal in the acoustic response signal at a level that allows acquisition of the resonance response signal above a predetermined signal to noise level.

BACKGROUND OF THE INVENTION

A. Available Gas Pipelines Monitoring System

Two main methods are available today for inspection/monitoring thestatus of the walls in gas pipe lines, namely, optical methods andmethods known as Magnetic Flux Leakage methods. Typically it is ofinterest to be able to determine the pipe wall thickness and otherconditions of the pipe during regular inspections, preferably undernormal operating conditions, and without having to take specialmeasures, such as e.g. filling the pipe with a liquid for the purpose ofproviding a coupling medium for performing such measurements byultrasonic means, since such special measures are costly and cause longdisruptions in the operation of the pipeline involved. Optical methodsare such as the one utilized by the “Optopig”, which is laser based andmeasures the distance to the wall with a resolution along and across thepipe wall of about 1 mm adapted to the inner surface, but does notmeasure the “remaining” thickness. The system is generally notapplicable for areas covered by condensate or other liquid material. TheMagnetic Flux Leakage method is a method which calculates the mass losswithin a given area, but is not able to calculate absolute thicknesses,and the method is not applicable for very thick pipe walls.

It has long been stated that non-contact ultrasound (NCU) measurementsof thickness and other characteristics of in a situation where a gasatmosphere exists between the measuring apparatus and the object to bemeasured generally is considered an impossible dream. In a pre-print ofa chapter for “Encyclopedia of Smart Materials”, editor A. Biederman,John Wiley & Sons, New York (expected in 2001), by Mahesh C. Bhardwaj,published on the world wide web pagehttp://www.ultragroup.com/Company/Publications/PDF/esmi1.pdf, thatgeneral view is emphasised. While some techniques for making NCUmeasurements are suggested in the aforementioned publication, they allappear to suffer from limitations to the extent that their commercialapplication and success in the market have not become apparent to thepresent applicants for patent.

Accordingly, there is a need for an apparatus and method that is simplein use, and that reliably and accurately provides NCU measurements ofthickness and other characteristics of an object to be measured in awide range of applications, and in particular for applications such asgas pipeline inspections.

B. Field of Invention

The present invention is particularly suitable for simultaneouslymonitoring gas pipelines for corrosion and characterize the mediumoutside the pipe wall. More particularly, the present invention relatesto a novel apparatus and method for the in situ monitoring of such gaspipes from the inside, and at the same time characterize the mediumsurrounding the pipe. If the pipe is coated, the characterization couldbe to decide whether the coating has loosened from the pipe wall or not.The method is also applicable with some geometric limitations if thereis a liquid layer covering the bottom of the pipe, the geometriclimitations relates to the critical angle between the gas medium and thewater surface. Above the critical angle all acoustic energy is reflectedfrom the surface, and measurements are not possible for angles largerthan this critical angle. One and the same apparatus is also applicablewithin the range of known offshore and onshore pipeline diameters (up toabout 1.50 m).

By insonifying the pipe wall with pulsed acoustic energy comprisingcomponents with wavelengths corresponding to twice the thickness of thewall, or integral numbers of this value, these frequencies will createstanding waves across the pipe wall. When the emitted pulse comes to anend, resonant energy is reradiated, and detected by a receivingtransducer located at a distance from the wall.

Referring to FIG. 1 this shows an example of an acoustic signal 100emitted from a transducer 111, reflected by a steel pipe wall 112 andreceived by a receiver transducer 111. Inside the pipe is a medium A,and outside the pipe is a medium B1. The acoustic signal 100 iscomprised of a direct reflected part 101 and a resonant part 102. Theamount of energy contained in the received signal, is influenced by theacoustic characteristics of the pipe wall as well as by the media onboth sides of the wall. The closer the acoustic impedance of the mediumbehind the wall is to the acoustic impedance of the wall, the lower isthe total reflected energy.

Referring to FIG. 2 this shows a corresponding result as in FIG. 1, onlymedium B2 is now different from medium B1 in FIG. 1, and as can be seenby comparing FIG. 1 and FIG. 2, the resonant part (102 and 202) of thereflected acoustic energy has changed.

SUMMARY OF THE INVENTION

The present invention provides an acoustic apparatus adapted to operatein a gas filled space and from a first side of an object to be measuredfor making a non-contact thickness measurement of the object to bemeasured or for making a non-contact characterisation of a mediumlocated on a second side of the object to be measured. Advantageously,the apparatus of the invention is embodied as an electro acoustic. Theapparatus typically comprises an electro acoustic transducer means,

a transceiver means coupled with the electro acoustic transducer meansand adapted to excite the electro acoustic transducer means to output anacoustic signal towards the object to be measured and receive anacoustic response signal there from, and

a signal processor adapted to process the response signal and todetermine on basis of the acoustic response signal a thicknesscharacteristic of the object to be measured. The electro acoustictransducer means of the invention has a transducer-to-gas acousticinterface, and the transceiver is adapted to operate the electroacoustic transducer means so as to emit into a gas filled gap betweenthe electro acoustic transducer means and the object to be measured anacoustic broad band pulse towards the object and to receive the anacoustic resonance response signal in the acoustic response signal at alevel that allows an acquisition of the resonance response signal abovea predetermined signal to noise level. The signal processor is adaptedto determine the thickness characteristic of the object to be measuredor to make a characterisation of a medium located on a second side ofthe object to be measured using a fast Fourier transformation (FFT) ofthe acquired resonance response signal above the predetermined signal tonoise level.

In an embodiment of the apparatus of the present invention, thetransceiver means coupled with the electro acoustic transducer means isadapted to operate with acoustic signals having acoustic components in afrequency range that is at least a decade lower than frequencies used intime of flight thickness measurements of the object to be measured.

In a further embodiment of the apparatus of the present invention, itincludes a transducer carrier means adapted to maintain the gas filledgap at a predetermined distance from a surface of the object facing thegas filled gap.

In a yet further embodiment of the apparatus of the present invention,the transducer carrier means is adapted to convey the electro acoustictransducer along the surface of the object facing the gas filled gap.

In a still further embodiment of the apparatus of the present invention,it is adapted to automatically establish the predetermined distance onbasis of at least one of a nominal thickness of the object to bemeasured, acoustic characteristics of the gas in the gas filled gap, andfrequencies of the broad band pulse, so as to optimise a quality of thenon-contact thickness measurement.

The present invention represents increased value for pipe lineinspection as it is able to measure from the inside of the at leastpartly gas filled pipe the absolute pipe wall thicknesses through a gaslayer as a coupling medium for an acoustic signal, the gas layer nowwith the employment of the apparatus or method of the present inventionis allowed to be in the range of less than or about 10 millimetres andup to 1000 millimetres or more, and simultaneously able to characterizethe medium located outside the pipe wall. It is also applicable in gaspipe lines with condensate present, and one and the same apparatus isapplicable for use in pipes of different diameters.

Further embodiments are readily understood from the following detaileddescription of the invention, and examples and the drawings used toexplain and disclose the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1B are schematic views of a pipe wall 110 where the media oneach side of the wall are similar, and the resulting reflected signal100 from this wall is also shown.

FIG. 2A-2B are schematic views of a pipe wall 210 where the mediuminside and outside of the wall are different, and the resultingreflected signal 200 from this wall is also shown.

FIG. 3 is a schematic view of the system of invention applied for gaspipe line inspection.

FIG. 4 is schematic view of a pipe with two coating layers.

FIG. 5 is a schematic view of one possible design of a transducer moduleand of one single transducer.

FIG. 6 is an outline of a design of a transducer module where thetransmitting array are spatially separated from the receiving array.

FIG. 7 is an outline of the transducer module in a pipe to illustratehow the distance between the corresponding transmitting and receivingtransducers depends on velocities of gas and sound, and distance to thepipe wall.

FIG. 8A-8F depict two arrays seen from different aspects.

FIG. 9 is a schematic view of one possible design of the system for gaspipelines monitoring system.

FIG. 10 is a schematic view of the algorithm of the invention.

FIG. 11 is a flow chart for harmonic set identification.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With Respect to the Overall System of the Invention

Referring to FIG. 3, this shows a schematic of an example of a scenariodepicting the use of the system according to the invention. A pipe isfilled with gas 300 with pressure in the range from 1 bar and upwards,for example natural gas transported from a reservoir. The system of theinvention employed in this scenario is preferably designed to map thecomplete thickness distribution of the pipe wall, and also characterizethe medium/media surrounding the pipe. The distance between thetransducers and the pipe wall can vary indicated by the arrows 305 and306 in dependency of a variation of the diameter of the pipe. Inside thepipe is a carrier device 301 for a large diameter pipe and 302 for asmaller diameter pipe, preferably an off-the-shelf pig, housing theultrasonic transducer array 307, and including the analog and digitalelectronics 307 and 308 and the power supply 309. The pig moves throughthe pipe driven by the gas flow.

Referring to FIG. 4, the pipe 400 is typically made of steel walls 401,and may be covered by one or more coating layers 402 and 403. The taskof characterizing the medium on the outside of the steel pipe wall couldfor example be to detect occurrences when the inner coating has loosenedfrom the steel pipe wall.

With Respect to the Transducers

Referring to FIG. 5, this shows a schematic of one half of a cylindricaltransducer module 500, and one of the transducers 532 comprising anumber of transmitting elements 532-1 and one receiving element 532-2.This design would be one of a number of possible designs. Thetransmitting transducer elements would typically be single crystalcomposites, while the receiving transducer elements would typically bepiezocomposite elements. The transducer may advantageously have one ormore matching layers in front of the transducer face (not shown in thefigure) to improve the coupling of acoustic energy at the interfacebetween the transducer and the gaseous medium due to the difference inacoustic impedance. Both the transmitting and the receiving elementswill be inside a housing that contains the electrodes coupled to each ofthe transmitting and receiving elements. Typically the housing also hasthe contact for electrically connecting the transducer to the electronicpart of the system.

Depending on the speed of the pig the transmitting and receiving part ofthe transducers could be spatially separated.

Referring to FIG. 6, this shows an arrangement where the receivingtransducer array 601-624 are spatially separated from the transmittingtransducer array 625-648. Depicted in the figure are the gas pipetransducer module 600, the distance between the arrays 650, one elementfrom the transmitting transducer array 647 as well as one element fromthe receiving transducer array 623. The design shown in FIG. 6 is onlyone of a number of possible designs.

Referring to FIG. 7 a pipe 700 with steel pipe walls 701 and thetransducer module 702 is shown. c is the speed of sound in the gasmedium inside the pipe, and v is the velocity of which the carrier pigis moving. The distance d1 between the corresponding transmitting 703and receiving transducers 704 depends on the velocity v of the gas, thesound velocity, c, and the distance from the transducer to the pipe walld2 according to the formula:d1=(2*d2*v)/c

The apparatus of invention contain a mechanical arrangement to changethe distance d1 according to the above formula.

The number of transducers will depend on the desirable coverage of thecircumference of the pipe wall, while the transmitting frequencyrelative to the speed of the gas decide the coverage in the lateraldirection. The transducers in the array could be operated individuallyor beam forming could be applied.

Referring to FIG. 8 depicting two arrays seen from two different aspectsin 801 and 802, and in 804 and 805. To make the illustrations simpleonly a static example is depicted. 803 shows the resulting acousticalinsonified areas (footprints) from each transducers of the array shownin 801 and 802. The coverage is less than 100%. Correspondingly thearrangement shown in 804 and 805 results in overlapping footprints and100% coverage as shown in 806.

With Respect to the Electronic System:

FIG. 9 shows a block diagram of the system shown in FIG. 3 used for insitu measurements of material properties of an object and the mediumbehind with a gaseous medium as the acoustic coupling medium used in thepresent invention.

A broadband electrical waveform is generated in a function generator901. In order to get the best signal to noise ratio possible, theamplitude of the broadband electrical waveform is increased using apower amplifier 902. When gas is used as coupling medium between thetransducers and the object, the transducers requires higher excitationvoltages compared to transducers coupled to media having higher acousticimpedance as e.g. water, in order to have the same signal to noiseratio. This is due to the large mismatch in acoustic impedance betweenthe gaseous media and the transducer, in addition to higher attenuationof acoustic energy in gas compared to e.g. water.

A transmitting matching network 903 is used to improve the systembandwidth. Such a matching network allows the power amplifier to driveover a wider band of frequencies within the required operationalbandwidth with improved linearity. The transducer and matching networkconstitutes a full section 3'rd order band pass filter. This could alsobe done with other matching network designs that constitute a band passfilter of higher orders.

The transmitting part of the transducer arrangement 904 converts thebroadband electrical waveform to mechanical vibrations. These mechanicalvibrations cause a broadband acoustical signal to propagate from thetransducer through the gaseous medium and to the pipe wall. At arrivalat the pipe wall, the broadband acoustical signal is partly reflectedfrom the wall and partly transmitted into the wall. If the partlytransmitted broadband acoustical signal comprises components withwavelengths corresponding to twice the thickness of the pipe wall, orintegral numbers of this value, these frequencies will create standingwaves across the pipe wall.

When the emitted pulse comes to an end, resonant energy is reradiatedand propagated thru the gaseous medium to be received at the receivingpart of the transducer arrangement 904. The receiving part of thetransducer converts mechanical vibrations to electrical signals normallyin the order of mV. Due to loss in signal strength through the cablebetween the receiving part of the transducer arrangement 904 and thedigitizer 906, these signals are applied to a low noise pre-amplifier905 before sending it through the cable. This pre-amplifier and isusually located right after the hydrophone. If the cable is long and/orthe amplitude of the signal is low, there could be a need of anadditional amplifier before the signal is going into the digitizer 906.

The amplified electrical signal is digitized by a digitizer 906 such asan analogue to digital (A/D) converter and stored in either the memoryof a processor or on a storage medium as e.g. a flash memory for lateranalysis. If the digitized data is stored in the memory of theprocessor, it could be analyzed, displayed and then stored. Theprocessor is using a technique described below in further detail.

The control unit 907 comprises a processor and could also include astorage medium.

One possible improvement of the system is to use equalizing techniqueson transmitting, receiving or both. The use of equalizing techniques canimprove the overall phase linearity, efficiency and amplitude responseof the system described in FIG. 9.

With Reference to the Algorithm

Throughout the flow chart of FIG. 10 it is assumed that displayed dataare also stored on a suitable storage medium.

1001 Time Signal

The series of real numbers corresponding to voltages from the DAQ unit180. Henceforth it will be referred to as the time vector.

1002 Input Parameters

-   -   Speed of sound of measurement object, c_(o)    -   Length of time window for spectral estimation, N    -   Expected width of primary echo without resonance tail, W    -   Spectral estimation methods    -   Speed of sound of liquid, c_(w)    -   Choice of window functions (e.g. Hanning, Bartley)    -   Sampling frequency, F_(s)    -   Frequency interval used in transceiver    -   Expected upper thickness boundary    -   Minimum ratio between peak energies in primary and secondary        echoes    -   Number of datasets in reference memory, M    -   Integer tolerance (1022-6)    -   Lower frequency weight threshold (1022-7)        1010 Time-Frequency Analysis

Inputs: time signal, spectral estimation technique, N, F_(s)

The power content in the time-frequency domain is estimated, using anystandard technique, such as the sliding Fourier transform, or the Wignerdistribution. The time of the maximum energy is identified, from thisand N the start time of the tail is found.

Outputs: matrix of power, vector of times (in units of samplinginterval), vector of frequencies (in Hz), start of tail time

1020 Identify Primary Echo

Inputs: time vector, expected width of primary echo

Finds the time corresponding to the largest pulse energy, and usesexpected width of primary echo to find the start and stop of the echo.

Outputs: start and stop times of echo

1021 Spectral Estimation

Inputs: time vector, spectral estimation method, start and stop timesfor analysis, window functions, N, F_(s)

The frequency power content of the time signal is estimated using anystandard technique, from periodogram based methods to parametricmethods, for example using the Yule-Walker model. The estimation isperformed in two windows, one comprising the tail only (starting at endof echo lasting to end of echo+N), and one comprising the echo and itstail, starting at the time start of echo−N lasting to end of echo+N.

Similarly, the bispectrum, the spectrum of the third-order cumulants, iscomputed using standard techniques. The interpretation of the bispectrumis less clear than for the ordinary spectrum, but its main advantagesare to reject Gaussian noise efficiently and to highlight phase-coupledfrequencies.

Outputs: power vector tail, vector with frequencies (in Hz)corresponding to the power values, power vector echo, vector withfrequencies (in Hz) corresponding to the power values, bispectrummatrix, corresponding frequencies

1022 Identification of Resonance Frequencies

Inputs: frequency vector tail, frequency vector echo, frequency vectorbispectrum, frequency interval used in transceiver

Identifies harmonic frequencies and assigns the correct harmonic orderto them. The procedure is detailed below under 1022-1 to 1022-8.

Outputs: index into time and frequency vectors corresponding to theresonance frequencies, harmonic orders

1023 Characterise Measurement Object

Inputs: c_(o), resonance frequencies, harmonic orders

The measurement object thickness is computed from

$\begin{matrix}{{d = \langle \frac{c_{o}n}{2f_{res}} \rangle},} & (2)\end{matrix}$where n is the integer indicating the harmonic order, f_(res) is theresonance frequency of harmonic order n, and <·> denotes averaging.

Display results.

Outputs: thickness estimates

1030 Identify Secondary Echo

Inputs: time vector, start and stop times of primary echo, minimum ratiobetween peak energy of primary and secondary echoes

The purpose is to determine whether there are two sets of echoessuperimposed in the time signal, which indicates that there is a liquidlayer between the transceiver and the measurement object. The secondaryecho is the part of the original transmitted pulse from the transceiverwhich is transmitted through the gas-liquid interface, proceeds throughthe liquid, is reflected from the measurement object, and finallytransmitted through the liquid-gas interface. Hence, the secondary echocontains the information from the measurement object and it is thereforecrucial that the further analysis is performed on this echo rather thanthe primary echo.

The secondary echo is assumed to have a similar temporal extent as theprimary echo, and to show up some time after the primary echo. If nosecondary echo is found, empty values are returned.

Outputs: start and stop times of secondary echo

1031 Is Liquid Present?

Inputs: start and stop times of secondary echo

If the inputs are empty, proceed the calculation with the primary echodetermining the windows used for analysis.

If the inputs are non-empty, liquid is deemed to be present and theanalysis proceeds using the secondary echo as the basis for determiningrelevant time windows.

Outputs: whether secondary echo was found

1032 The Depth of the Liquid Layer

Inputs: times of secondary and primary echoes, c_(w)

From the time difference between the secondary and primary echoes, thedepth of the liquid layer is computed from

$l = \frac{c_{w}}{t_{\sec} - t_{prim}}$with t_(sec) and t_(prim) being the time of arrival of the secondary andprimary pulses, respectively.

Stores the value and displays it.

Outputs: estimated depth of liquid layer

1040 Decay Times

Inputs: time-frequency power matrix, indices of resonance frequencies,start of tail time

The characteristic decay times of the resonance frequencies in the tailis found.

Outputs: the decay times of the resonance frequency

1041 Energy of Resonance Frequencies

Inputs: power vector tail, power vector echo, indices of resonancefrequencies

Outputs: The ratio of the power in the resonance frequencies by thetotal power (power spectral density integrated with respect tofrequency) in the echo pulse.

Now, details of the flow chart of FIG. 11 for harmonic setidentification is explained.

1022-1 Find Local Maxima/Minima

Inputs: power vector echo, power vector tail, bispectrum vector

Finds local maxima in the bispectrum vector and the power vector tail.Finds local minima in the power vector echo.

The union of the three sets is the list of potential harmonic frequencycandidates.

Outputs: harmonic frequency candidates

1022-2 Weighting of Maxima/Minima

Inputs: harmonic frequency candidates, power vector echo, power vectortail, bispectrum vector, filter size

-   -   1. Initialize weights vectors with values zero except at        harmonic frequency candidates, where the value from the power        vectors is used for bispectrum and tail. The weight vectors are        normalized to the largest value in each case, e.g. all weights        from the bispectrum candidate frequencies are normalized to the        maximum value in the bispectrum vector.    -   2. subtract the power vector echo with its filtered version. The        difference at the local minima defines the weight in this case.        Normalise to the largest difference found.    -   3. One now has available three sets of weights, W_(bisp),        W_(tail), W_(echo), each normalized so the largest weight is 1.    -   4. For each set, scale the weights by

${W_{j}(i)} = {{W_{j}(i)}{\prod\limits_{k \neq j}{\exp( {- d_{k}} )}}}$

-   -    where d_(k) is the shortest distance to a non-zero weight in        set k. W_(j)(i) is the ith element of the jth set.    -   5. sum the weights from each set to obtain a single weight        vector

The ensuing weight vector gives weight to large peaks/deep minima in therespective power vectors, but penalizes each weight if it is far fromfrequencies in the other sets. Weights are real numbers between 0 and 1.

Outputs: weights assigned to each harmonic frequency candidate

1022-3 Sort According to Weights

Inputs: weights, harmonic frequency candidates

Sorts the weights vector, and uses the sort indices to rearrange theharmonic frequency candidates so that they are listed in decreasingweighted order.

Outputs: sorted harmonic frequency candidates

1022-4 Build Frequency Sets

Inputs: sorted harmonic frequency candidates, weights, frequency weightsthreshold

-   -   1. Reject all candidate frequencies below the threshold    -   2. Rearrange frequency candidates into sets. If there are N        candidates, then build N lists {f₁, . . . , f_(N)}, {f₁, . . . ,        f_(N-1)}, and so on, where the smallest weighted frequency in        the previous list is progressively removed. Each list is        henceforth known as a frequency set.

Each frequency set is denoted F_(n).

Outputs: frequency sets {F₁, F₂, . . . , F_(N)}

1022-5 Loop Through all Sets, i=1, . . . , N

1022-6 Find Harmonic Sets:

Input: Frequency sets {F₁, F₂, . . . , F_(N)}, integer tolerance,expected maximum thickness, frequency interval used in transceiver

The harmonic sets for one frequency list F_(i) is computed as follows:initially a n×n matrix with filled with all possible ratios offrequencies is found,

$M_{ij}^{\prime} = \frac{f_{i}}{f_{j}}$

The matrix M′ is used to build a larger matrix M by concatenating kM′,k=1,2, Λ, k_(max) as follows:

$M = \begin{bmatrix}{1 \cdot M^{\prime}} \\M \\{k_{\max} \cdot M^{\prime}}\end{bmatrix}$

The integer k_(max) is computed from the maximum allowed thickness, auser input.

The next step is to round all elements in M to their nearest integer,and compare the difference between the integer values and the frequencyratios in M. An element is deemed an integer if this difference is lessthan a user specified threshold, typically 0.1, and a matrix N where allnon-integer elements in M equal zero if found. The rows in N identifythe harmonic sets: for a given N_(ij) element the value corresponds tothe harmonic order of frequency f_(j) in the frequency list.

Output: Set of integer matrices {N₁, N₂, . . . , N_(N)}.

1022-7 Removing Elements in N_(n):

Input: Set of integer matrices {N₁, N₂, . . . , N_(n)}, expected maximumthickness, frequency interval used in transceiver

The harmonic order matrices N_(n) are significantly reduced by removingrows containing a value above the max order k_(max). All duplicate rowsare removed, and rows giving a thickness above the user input maximumvalue are removed.

Outputs: Set of reduced integer matrices {N₁, N₂, . . . , N_(n)}.

1022-8 Count Number of Harmonics in N_(i):

Input: Set of reduced integer matrices {N₁, N₂, . . . , N_(n)}.

For each N_(n), the harmonic set with the largest number of uniquefrequencies are recorded. The numbers are stored in a vector Φ.

Output: Vector Φ of maximum number of unique sets.

1022-9 Finding Optimum Subset of Frequencies:

Input: Vector Φ of maximum number of unique sets, number of frequenciesin each frequency set, set of reduced integer matrices {N₁, N₂, . . . ,N_(n)}.

The aim is to find the optimum subset of the original frequency list.Each subset is associated with a number of unique harmonics stored in Φ.In addition, each subset has a number of frequencies.

The optimal subset if found by finding the highest ratio of Φ divided bythe number of frequencies in the list, neglecting the trivial case foronly a single frequency. In this process we have accomplished both arejection of frequencies, and obtained harmonic sets.

Output: Indices to optimal subset of frequencies, set of harmonics.

OTHER APPLICATIONS

So far the system of invention has been described as a pipe scanner, butthe system of invention could also be applied as a hand held device inair. For this purpose the device could contain a single transducersystem if the application mode is spot checks. For scanning purposes anarray would most likely be appropriate. The application areas could bespot checks/scanning of ship hulls from the inside or onshore pipes andstorage tanks from the outside. Instead of applying the system ofinvention for thickness scanning of pipe walls or containers, the samesystem will be applicable for characterizing pipe walls if the thicknessand sound velocity of these walls are known. This characterization couldbe to detect deviations from a perfect pipe wall. One example would beinside characterization scanning of risers. Another application will bewell logging/down-hole inspection during production. The casingthickness will be measured, as well as characterization of the mediumoutside the casing, e.g. differentiate between concrete gas or fluid.

REFERENCES

-   International Publication Number: WO 01/83122 A1—Method and    apparatus for equalizing transfer functions of linear    electro-acoustic systems.-   Mahesh C. Bhardwaj: “Non-contact ultrasound: The last frontier in    non-destructive testing and evaluation”, published on the world wide    web page http://www.utragroup.com/Company/Publications/PDF/esm1.pdf

1. An electroacoustic apparatus adapted to operate in a gas filled spaceand from a first side of an object to be measured for making anon-contact ultrasound thickness measurement of the object to bemeasured or for making a non-contact ultrasound characterisation of amedium located on a second side of the object to be measured, theapparatus comprising an electroacoustic transducer means, a transceivermeans coupled with the electroacoustic transducer means and adapted toexcite the electroacoustic transducer means to output an acoustic signaltowards the object to be measured and receive an acoustic responsesignal therefrom, and a signal processor adapted to process the responsesignal and to determine on basis of the acoustic response signal athickness characteristic of the object to be measured, wherein theelectroacoustic transducer means has a transducer-to-gas acousticinterface, the transceiver is adapted to operate the electroacoustictransducer means so as to emit into a gas filled gap between theelectroacoustic transducer means and the object to be measured anacoustic broad band pulse towards the object and to receive the anacoustic resonance response signal in the acoustic response signal at alevel that allows an acquisition of the resonance response signal abovea predetermined signal to noise level, and the signal processor isadapted to determine the thickness characteristic of the object to bemeasured or to make a characterisation of a medium located on a secondside of the object to be measured using a fast Fourier transformation(FFT) of the acquired resonance response signal above the predeterminedsignal to noise level.
 2. The apparatus of claim 1, wherein thetransceiver means coupled with the electroacoustic transducer means isadapted to operate with acoustic signals having acoustic components in afrequency range that is at least a decade lower than frequencies used intime of flight thickness measurements of the object to be measured. 3.The apparatus of claim 2, wherein it includes a transducer carrier meansadapted to maintain the gas filled gap at a predetermined distance froma surface of the object facing the gas filled gap.
 4. The apparatus ofclaim 2, wherein it is adapted to automatically establish thepredetermined distance on basis of at least one of a nominal thicknessof the object to be measured, acoustic characteristics of the gas in thegas filled gap, and frequencies of the broad band pulse, so as tooptimise a quality of the non-contact thickness measurement.
 5. Theapparatus of claim 1, wherein it includes a transducer carrier meansadapted to maintain the gas filled gap at a predetermined distance froma surface of the object facing the gas filled gap.
 6. The apparatus ofclaim 5, wherein the transducer carrier means is adapted to convey theelectroacoustic transducer along the surface of the object facing thegas filled gap.
 7. The apparatus of claim 6, wherein it is adapted toautomatically establish the predetermined distance on basis of at leastone of a nominal thickness of the object to be measured, acousticcharacteristics of the gas in the gas filled gap, and frequencies of thebroad band pulse, so as to optimise a quality of the non-contactthickness measurement.
 8. The apparatus of claim 5, wherein it isadapted to automatically establish the predetermined distance on basisof at least one of a nominal thickness of the object to be measured,acoustic characteristics of the gas in the gas filled gap, andfrequencies of the broad band pulse, so as to optimise a quality of thenon-contact thickness measurement.
 9. The apparatus of claim 1, whereinit is adapted to automatically establish the predetermined distance onbasis of at least one of a nominal thickness of the object to bemeasured, acoustic characteristics of the gas in the gas filled gap, andfrequencies of the broad band pulse, so as to optimise a quality of thenon-contact thickness measurement.